A Passive Optical Networks (PON) system is one of the standards for access optical communication systems widely used to achieve public communication network using an optical fiber.
A PON system is constituted by a single Optical Line Terminal (OLT) which is a device on a provider station side, and constituted by multiple Optical Network Units (ONUs) which are subscriber terminal devices connected to the OLT via an optical star coupler.
Distances from an optical star coupler to ONUs vary for each ONU. The optical power of optical signals received by an OLT via an optical star coupler also varies for each ONU. Due to this, an OLT requires a receiver that provides a large dynamic range in order to receive optical signals as the optical power is varied.
A receiver having a large dynamic range, in general, employs Automatic Gain Control (AGC) to equalize amplitudes of converted voltage signals by implementing a gain (a conversion gain of a voltage over an input current) setting that is high when optical power of received optical signals is low, while implementing a low gain setting when the optical power of received optical signals is high.
There are various standards for the AGC. Patent Literature 1, for example, discloses a gain control method (feedback control method) for a transimpedance amplifier that uses continuous variance of the value of a feedback resistor (a resistor that defines a transimpedance amplifier gain) in the transimpedance amplifier, based on an average voltage value that is output from the transimpedance amplifier. Patent Literature 2, on the other hand, discloses a gain control method (feedforward control method) which directly uses a current that corresponds to the optical power in optical signals for the light received by a photodiode, to continuously vary the value of a feedback resistor in a transimpedance amplifier.